This invention relates to packaging materials and particularly to conformable films which are microwave susceptive and useful for food packaging and microwave cooking.
There has been much interest recently in packaging materials which aid in browning and crispening of food items in a microwave oven. U.S. Pat. No. 4,267,420, Brastad, discloses a food item wrapped with plastic film having a very thin coating thereon. An additional sheet or film of plastic is optionally laminated to the coating for abrasion protection. Other exterior support by more rigid dielectric materials such as paperboard and the like is also disclosed. The coating converts some of the microwave energy into heat which is transmitted directly to the surface portion of the food so that a browning and/or crispening is achieved.
U.S. Pat. No. 4,641,005, Seiferth, discloses a disposable food receptacle for use in microwave cooking, which includes a provision to brown the exterior of the food in the receptacle. A thin layer of an electrically conductive material is incorporated into the receptacle on the food contacting surfaces thereof, so that the conductive layer will become heated by the microwave radiation and will, in turn, brown the exterior of the food in the receptacle. The receptacle includes a smooth surfaced plastic film, as a protective layer, and a support means formed of paper stock material.
U.S. Pat. No. 4,713,510, Quick et al., discloses a microwave ovenable package including a layer of material that will convert a portion of the microwave energy to heat and a layer of paperboard interposed between the energy-converting layer and the food. The energy-converting layer may be carried on a plastic film, and an additional layer of paperboard may be used to sandwich the energy-converting layer and the plastic film between layers of paperboard. For the purpose of providing a more intense heating effect, two energy-converting layers, each on a dielectric substrate, sandwiched together between layers of paperboard, are disclosed.
U.S. Patent application Ser. No. 07/188,556 discloses a conformable multilayer structure useful for packaging food for microwave cooking. The structure comprises at least one layer of flexible, heat resistant, microwave transparent film, e.g., PET; at least one layer of flexible, heat resistant, heat stable, microwave transparent plastic film, e.g., heat stabilized PET or cellophane; and at least one layer of substantially continuous microwave susceptor material located on a surface of a film of the laminate. The various layers of the laminate can be held together by adhesive, e.g. a crosslinkable copolyester.
One persistent problem in microwave film structures is overheating and melting of areas of the film which are, for example, not in contact with food as a heat sink or which represent overlapping areas with multiple thicknesses of susceptor material. Several attempts have been made to avoid this problem. U.S. Pat. No. 4,735,513, Watkins et al., Apr. 5, 1988, discloses a flexible, sheet structure comprising a base sheet of e.g. polyester, having a microwave coupling layer. The sheet may be laminated to a backing sheet of dimensionally stable flexible material transparent to microwaves. Examples of such backing sheets include a synthetic sheet formed from synthetic plastic fibers of a non-thermoplastic and dimensionally stable composition, or, preferably, paper. The microwave coupling layer is present as an island covering selected areas of the sheet. The uncoated portions will not be heated and will not be damaged by microwave energy.
European patent application 0 276 654, published Aug. 3, 1989 discloses a method of making a susceptor material in which a liquid component carrying distributed microwave interactive particles is dried on a receiving surface such as paper or film. A protective layer may cover the distributed particles interactive layer after drying. The liquid component comprises a cross-linking synthetic resin, cured for example by subjecting it to heat to fix the particles in their distributed condition.
Other approaches to solve this problem are illustrated in U.S. Patent application Ser. No. 07/245,153, now U.S. Pat. Nos. 4,851,632 and 07/377,159. The former application discloses a microwave cooking package in which an insulating layer is provided between regions of the film laden with susceptor which are in proximity with each other, thereby preventing such regions from overheating. The latter application discloses a package in which seal areas are protected from overheating by means of a reflective shield.
In spite of extensive work in this area, there remains a need for microwave susceptor films, and particularly microwave susceptor films which are conformable and preferably transparent, which do not melt, shrivel, shrink, or otherwise deform in seam areas or in areas which are not proximate to food as a heat sink. The present invention provides such a film.